Generally, an on-vehicle electric power source system includes an electric power source and a high-voltage circuit connected to the power source in parallel. The electric power source and the high-voltage circuit are both connected to an actuator (e.g., an electric motor used for an electric motor-assist power steering (EPS) system or an electric brake). Only when a steering wheel is turned to a left turn limit or to a right turn limit and thus a great steering-assist torque, in other words, a large electric power, is required, a voltage rise or a boost-up operation is made by means of- the high-voltage circuit, as required for an adequately large electric power. One such on-vehicle electric power source system has been disclosed in Japanese Patent Provisional Publication No. 2005-261180 (hereinafter is referred to as “JP2005-261180”).